The present invention relates to a MOSFET device, and more specifically to an arrangement or structure suitable for protection of a power MOSFET against overcurrent or overheating.
FIGS. 35-38 show a conventional example of a vertical MOSFET device disclosed in IEEE Power Electronics Specialists Conference Record, 1985, pp 229.
FIG. 37 is a block diagram of the entire circuit of this device. A vertical power MOSFET is integrated with a protective circuit comprising CMOS and bipolar IC into a so-called power IC of a single chip. The device of FIG. 37 includes a thermal limit circuit 89 for detecting excessive temperature and a current limit circuit 88 for detecting excessive current. If an excessive condition is detected by the limit circuit 88 or 89, a signal is sent to a section of CMOS logic, and the power MOSFET 81 labeled as POWER TMOS is turned off to protect the device.
The current limit is shown in FIG. 35. The current limit has a single cell MOSFET 82 and a current sensing resistor 83. The MOSFET 82 consists of a single cell whereas the main MOSFET 81 consists of several thousand cells (3000 cells in this example) which are all connected in parallel. Therefore, the current through the main MOSFET 81 is 3000 times as large as the current through the single cell MOSFET 82.
A main current flowing through a load 84 is monitored by a current mirror circuit composed of the single cell MOSFET 82 and the current sensing resistor 83.
When the voltage drop across the current sensing resistor 83 is increased by an increase of the current flowing therethrough, either an upper rail comparator 85 or a lower rail comparator 86 produces an overcurrent detection signal. This signal is sent to a gate driving circuit, and causes the interruption of current. In this way, the current limit circuit prevents damage of the device due to overcurrent.
However, this MOSFET device is complicated in circuit configuration, and requires a wide variety of devices such a CMOS logic. Therefore, the size of a power IC chip is increased, the fabricating process is complicated, and the cost is high. Furthermore, this conventional device has not only the overcurrent protection but also other protective functions against excessive temperature and overvoltage, so that cost-utility is low in cases where only overcurrent protection is required.
FIG. 38 shows the thermal protection circuit of this conventional device. This circuit is arranged to detect a change of a base-emitter voltage of a bipolar transistor due to temperature change, and perform the protective function in accordance with the result of comparison with a reference voltage. However, in order to perform such a delicate analog control accurately, this circuit has a complicated and extensive configuration.
FIG. 36 is a cross section showing a basic structure of this conventional device. This structure requires a complicated and time-consuming fabrication process including two steps of epitaxial growths, and a step for forming a buried layer.